In submerged or immersed membrane filtration systems aeration using gas, typically air, is often employed on a continuous basis to control fouling. This is particularly the case for wastewater bioreactors. Other prior art systems utilise cycles of high gas flows followed by a lower gas flow or no gas flow. Mixtures of liquid/gas have also been used to scour the membranes and provide a reduction in gas consumption.
There are several problems with the cycling approach:
1. The cycle frequencies are typically very high, as it is not good to have the gas off in the process for too long as fouling increases rapidly during this phase. High cycle frequencies increase wear on valves that are required to cycle between multiple cells or membrane tanks, so as to keep the overall gas flow more or less constant from the blower. There are also practical limits to how fast the valves can be cycled between on and off caused by limitations of valve design and the time it takes to accelerate the gas flow.
2. When the gas is off for one cell it provides an opportunity for sludge to flow back into the aerators which can cause clogging of the aerators over time.
3. The longer the time between cycles the more likely it is for fouling to increase and the sludge to go anoxic.